Most military aircraft contain an ejection seat that allows the pilot to escape the aircraft while in flight. When an ejection seat is jettisoned from the cockpit of an aircraft, it must pass through the region occupied by the transparent canopy of the aircraft. In instances where the canopy is not jettisoned prior to the ejection seat firing, the ejection seat must be capable of blasting entirely through the canopy. To reduce the risks to the pilot or other aircraft occupant attendant to forcing the-ejection seat through the canopy, canopy fracture systems have been provided to fracture the canopy and better clear a path for the ejecting occupant so as to minimize bodily impact with the canopy.
Canopy fracture systems have been effective at removing portions of canopies that are made from fragilizing materials, such as cast or stretched acrylic. Fragilizing materials are those that may be caused to shatter into a significant number of pieces on application of sufficient pressure or explosive force. These systems utilize a mild detonating charge (MDC) or linear shaped charge (LSC) placed on, in or near the transparency which, upon detonation, creates shock waves that fracture the canopy. With fragilizing canopies, such as those made from cast or even stretched acrylic, it is not necessary to fully sever the material in order to defeat its structural integrity.
Many high performance aircraft utilize polycarbonate in their canopies instead of acrylic. Polycarbonate is a nonfragilizing material, meaning that it does not shatter on application of explosive force. With polycarbonate, it is absolutely necessary to fully sever the material because fracturing to complete the break is very unreliable. Because polycarbonate has a relatively low melt point and because cutting the material generates considerable heat there also exists a potential for resealing behind the cut if the severance is not complete.
U.S. Pat. No. 5,170,004 teaches an explosive device wherein a nearly incompressible transmitting medium is placed between the explosive device and an aircraft canopy. The function of the transmitting medium is to transmit the shock wave, produced upon detonation, to the canopy with a minimum of dissipation. This device is effective on fragilizing canopies but has not been successful with thicker non-fragilizing canopies made from monolithic polycarbonate, polycarbonate laminates or acrylic/polycarbonate laminates.
U.S. Pat. No. 5,780,763 teaches a method of fracture wherein explosive cords are placed in parallel grooves on the tipper surface of a canopy and simultaneously detonated to create overlapping shock waves. This method is apparently capable of breaking a 0.75 inch thick polycarbonate in the laboratory at ambient temperature or below, but is unreliable at elevated temperatures on the order of 165° Fahrenheit or above. This method, however, requires two charges, grooves to be cut in the material and also utilizes shock waves which are not reliable on polycarbonate materials.
U.S. Pat. No. 5,954,296 also relates to an aircraft canopy fracture system. The 296 patent claims a canopy with a severable region shaped so as to inhibit passage of the severable region back through the canopy after severance. The 296 patent also refers to the use of a LSC to sever a polycarbonate canopy.
All documents, including other patents and references, referred to in this document are hereby incorporated by reference in their entirety, although no documents are admitted to render any of the claims unpatentable either alone or in combination with any other references known by the applicant.
The prior art also does not adequately deal with the problems of providing for severance around the corners of the severable portion of the target or for routing a charge over the top of another charge.
It is therefore an object of this invention to provide a severance method which severs materials such as polycarbonate, polycarbonate laminates or acrylic/polycarbonate laminates that cannot be severed by existing methods except by use of excessive amounts of explosive charge or by placing the charge inside the material to be fractured, thereby degrading the material's structural integrity.
It is another object of this invention to provide a severance method that allows for a minimum amount of explosive charge to sever a given thickness of material at high and low temperature extremes.
It is further an object of this invention to provide a severance method that allows for severance around corners and through intersecting portions of the explosive charge.